1. Field of the Invention
The present invention relates to a method of fabricating a microelectronic device with programmable memory, as well as to a microelectronic device with programmable memory, obtained from said method.
2. Description of Related Art
Microelectronic devices with programmable memory are typically, but not exclusively, programmable ionic conduction (metallization) cells, which are computer memories known as “non-volatile” computer memories. Such programmable ionic conduction cells are well known by the acronyms CBRAM, standing for “conductive-bridging random access memory”, or PMC, for “programmable metallization cell”.
That type of microelectronic structures (CBRAM or PMC) is well known to the skilled person and has been described in document U.S. Pat. No. 6,084,796, for example,
A CBRAM (or PMC) typically comprises a vertical stack of layers formed by a substrate based on a silicon type semiconductor on which the following layers are deposited in succession: an electrode termed the bottom electrode, a layer of a chalcogenide glass doped with silver (i.e. solid electrolyte), and an electrode termed the top electrode formed from silver. The layer of a chalcogenide glass is thus interposed between the bottom electrode and the top electrode.
Said electrodes are configured to cause a metallic dendrite to grow (i.e. formation of an electrically conductive bridge) from the negative of the two electrodes towards the positive of the two electrodes through the layer of doped chalcogenide glass when a voltage is applied between said electrodes. By applying an opposite voltage between said two electrodes, the reverse phenomenon is obtained, namely disappearance of the metallic dendrite (i.e. disappearance of the electrically conductive bridge) within the doped chalcogenide glass layer.
Thus, when the electrically conductive bridge is created (the step known as “writing”), the logic state of the device may be represented by “1”, or may correspond to the “ON” state, whereas when the electrically conductive bridge disappears, the logic state of the cell may be represented by “0” or may correspond to the “OFF” state.
A first function that may be desired in CBRAMs is to have a microelectronic structure with a retention time (for information in the memory) that is as long as possible, which may in particular be induced by a compact layer of a chalcogenide glass. Once the electrically conductive bridge has been formed by applying a voltage between the two electrodes, the retention time corresponds to the lifetime of the electrically conductive bridge when said voltage is no longer applied.
A second desired function in CBRAMs is to have a microelectronic structure with higher electrical yield. The electrical yield of the memory structure may depend on the stoichiometry of the chalcogenide material, or in other words on the atomic percentage of the various elements that make up the chalcogenide material. Said stoichiometry is an essential factor in obtaining optimized electrical performances in programmable ionic conduction cells. As an example, when considering a chalcogenide based on germanium and sulfur with formula GexS100-x in which x is an integer, the higher the sulfur content relative to the germanium, the better the electrical performance of programmable cells formed from that chalcogenide. An example of a chalcogenide with a high stoichiometry that may be mentioned is GeS2. That particular type of stoichiometry has several advantages. It can improve the thermal stability of the chalcogenide and increase the solubility point of the doping metallic element in the chalcogenide during fabrication of said programmable cells, and thus increase the electrical performances of said cells. Thus, the more “stoichiometric” the chalcogenide, the higher can be the electrical yield of the microelectronic CBRAM structure.
However, it is known that a CBRAM microelectronic structure with a high yield, or in other words with a nigh stoichiometry, can in particular exhibit high mobility of silver ions within the layer of doped chalcogenide glass. However, high mobility of silver ions tends to perturb the stability of the electrically conductive bridge, thus the stability of the retention time.